365 nm photon-induced dynamics of ClNO adsorbed on MgO(100)

Abstract

Temperature programmed desorption (TPD) and 365 nm photolysis of ClNO adsorbed on MgO(100) at 90 K were investigated under ultrahigh vacuum conditions. The crystal was treated in a way that largely eliminated oxygen vacancies and yielded a relatively smooth surface. Angularly resolved time-of-flight (TOF) mass spectra and state-selective resonance-enhanced multiphoton ionization (REMPI) spectra of NO photoproducts were obtained. The TPD data indicate that ClNO desorbs at surface temperatures above 160 K for exposures (Θ) below 0.2 Langmuirs (L), while for higher values of Θ the main desorption peak is near 120 K. The higher temperature feature, which saturates at Θ∼0.3 L, is probably associated with binding to defect sites. Thermal desorption is believed to be molecular at all coverages. Irradiation at 365 nm for 0.1≤Θ≤5.0 L yields products having low average translational energies and broad translational energy distributions. NO fragment REMPI spectra were recorded at Θ≥0.7 L. The rotational distributions could be fit with a temperature of 110±10 K, i.e., comparable to that of the substrate. These results differ from those obtained in the photodissociation of gas-phase ClNO, where the NO fragment has high translational and rotational energies. However, the present results are similar to those obtained on rougher MgO(100) surfaces. Possible mechanisms are discussed.